How to interpret arterial blood gases test report (ABGs Report Interpretation)

(Arterial blood gases) ABG report tells us about the level of hypoxemia and acid-base balance. A medical student must know how to interpret arterial blood gases (ABG Report) correctly and accurately and he should also know the procedure of taking the sample for arterial blood gases test.

Standard Procedure:

To take the arterial blood sample;
1. first of all heparinize the syringe with 0.1 ml heparin to prevent clot formation.
2. Draw blood from the radial or brachial artery (femoral artery if none of these is accessible). Prevent Air bubble otherwise, it will affect the results.
3. Immersed the sample in an ice bag immediately to prevent metabolism that can decrease PO2 and increase PCO2

Normal Values:

pH: 7.35-7.45
PO2 (partial pressure of oxygen): 75-100 mm of Hg
PCO2 (partial pressure of carbon dioxide): 36-46 mm of Hg
HCO3 (bicarbonate ion): 22-26 mmol/L
O2 saturation: 95-100% (oxygen saturation below 90 per cent produces cyanosis in a person who has a normal level of haemoglobin)

When you see the ABG report, first of all, see the PH whether it is acidic or basic. If PH is less than 7.35 it is acidosis and if it is more than 7.45 it is alkalosis. Now after figuring out acidosis or alkalosis, see if it is respiratory acidosis or alkalosis or it is metabolic acidosis or alkalosis and after that see if metabolic or respiratory acidosis/alkalosis has been compensated by the body or not. 

Primary respiratory acidosis.

In primary respiratory acidosis, there is retention of CO2 which results in an increase in PCO2. More PCO2 means more acid in the body. it has a respiratory cause, for example, COPD (chronic obstructive pulmonary disease) or hypoventilation ( type II respiratory failure )
In primary acidosis: the result will be as follows:

- Low pH ( which confirms acidosis )
- High PCO2 (which Confirms CO2 retention)
- Normal HCO3 (which confirms normal metabolism)

Compensated Respiratory Acidosis.

In compensated respiratory acidosis:

- pH will be normal, which will tell us that the body has successfully compensated the acidosis:
- PCO2 will be high ( which will tell us that acidosis was respiratory in origin )
- Raised HCO3, to neutralize the raised H+ ions, kidneys start retaining HCO3 ions (the basic component)

Primary Respiratory Alkalosis:

In primary respiratory alkalosis:

- pH will be high, so it is alkalosis
- PCO2 will be low, due to hyperventilation, hyperventilation will remove the carbon dioxide from the lungs and in turn from the body. Hyperventilation can be pathological or intentional.
- HCO3 will be normal in cases where compensation hasn’t yet started, if compensation starts, its level decreases.

Compensated Respiratory Alkalosis:

As alkalosis is respiratory in origin, so the body has to decrease the level of HCO3 by excreting it through kidneys, to normalize the pH
Therefore, the picture will be;

- Normal pH ( as compensation has occurred)
- Low PCO2
- Low HCO3

Primary metabolic acidosis:

- Low pH ( so it is acidosis )
- Normal pCO2 ( so it is not respiratory in origin )
- Low HCO3 ( so acidosis occurred due to low level of base in body )
- Causes are chronic renal failure, Diabetic Ketoacidosis and lactic acidosis

Compensated Metabolic acidosis:

In metabolic acidosis, lungs try to blow out carbon dioxide (acid) to maintain the ratio of acid and base in the body (by hyperventilation). so pH will come toward the normal range.
so the picture will be:

- Normal pH
- Low pCO2
- Low HCO3

Metabolic Alkalosis:

In metabolic alkalosis picture will be:

- high pH
- Normal (or near normal if compensation has just started) pCO2
- High HCO3

Compensated Metabolic Alkalosis.

in compensated metabolic alkalosis lungs will try to retain CO2 in the body (CO2 = acid), so PCO2 will be high
so the picture will be

- Normal pH
- high PCO2
- High HCO3

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